Instead, the bloody conflict that for nearly a year has defied most world appeals for peace exploded in their faces, trapping them in Sarajevo, the savaged capital of Bosnia-Herzegovina. For days, they have been jeered, shelled, sniped at and even held at gunpoint, mostly by Serb irregular militias.

``I regret leaving Sarajevo, because it`s not a humiliation for the UN, it`s a failure for peace,`` said a discouraged French Gen. Philippe Morillion, deputy commander of the multinational UN Protection Force (UNPROFOR) that had tried to set up its headquarters there.

The bulk of that command center, nearly 300 soldiers, withdrew this weekend. Western diplomats in Belgrade, the capital of Serbia and the newly diminished Yugoslavia, said the setback has troubling implications for the credibility and role of the UN in a post-Cold War world struggling to come to terms with the violent escalation of ethnic nationalism.

``Will international peacekeeping itself be discredited?`` asked one foreign analyst. ``If this situation is too difficult, will the UN only be confined to minor wars and negotiating minor cease-fires?``

The immediate impact of the UN exodus was felt by the helpless civilians still trapped in Sarajevo, a city of 560,000 with minimal electricity, food and water supplies. Serb women standing in their gardens on the outskirts of the city wept openly as one UN convoy pulled out this weekend.

Some 1,300 people have died and 700,000 have been left homeless since war broke out in ethnically mixed Bosnia after the former Yugoslav republic declared independence two months ago over the objections of its large Serb minority. Finally, even UN forces became targets.

``It was terrible to see the way things have gone. We had the impression that the Bosnian population was committing suicide,`` Morillion said.

The UN Sarajevo headquarters was shelled in recent days, apparently by Yugoslav army artillery, the peacekeepers said. UN flak jackets and ammunition were taken by the reckless ethnic militias running rampant there and manning dozens of heavily armed checkpoints along the road back to Belgrade.

``When we arrived, Sarajevo was a hustling, bustling city,`` said British Maj. Tom Jefferson, 38, assistant force medical officer who went in with the first contingent on March 22. ``It`s now a place like what I imagine Beirut to be, though I`ve never been there.``

Much of the force was confined to a bomb shelter below the Hotel Stoicevac and the basement of an apartment house on the outskirts of Sarajevo for 48 hours starting last Thursday, he said. Up above, fighting and shelling went on for two days between Muslims and Serbs.

When they tried to move to another location, Jefferson said, Serb militias held UN forces at gunpoint in tense standoffs, ordered them to help collect the bodies of dead Serbs and even poked guns in their ribs. The situation in Bosnia was so tense, he said, he started smoking cigarettes again after having stopped for 12 years.

``You are causing me too much trouble,`` one Serb militiaman told Danish Col. Svend Harders, UN chief of operations, pushing him with a gun barrel during one tense negotiation.

Amazingly, only one UN soldier was wounded, and he only slightly. Nevertheless, continuing the peace mission under such conditions will be

``impossible`` for now, Morillion told reporters after the first UN convoy with nearly 50 vehicles and 200 military and civilian personnel arrived in Belgrade early Sunday morning.

The trip, usually four hours, took 15 hours because of all the checkpoints. A full moon illuminated the white UN vehicles, some of which had bullet holes or missing windows from the fighting.

Later Sunday another UN contingent of about 80 people in 40 vehicles left Sarajevo during a lull in the fighting. Heavier vehicles headed for Zagreb, the Croatian capital. The rest, including UNPROFOR commander Lt. Gen. Satish Nambiar and his staff, came to Belgrade.

Sarajevo had been chosen as the headquarters of the 14,000-member multinational peacekeeping force deployed in neighboring Croatia to separate Serbs and Croatians after their bloody civil war late last year claimed some 10,000 lives.

Croatia`s tragedy then made Sarajevo seem a tranquil, more neutral haven. Its rich mix of ethnic Serbs, Croatians, Slavic Muslims, Jews and other minorities had made it exemplary of the old Yugoslavia.